Scientists using NASA's Wide-field Infrared Survey Explorer (WISE) say that most centaurs are comets.
These small celestial bodies orbiting the sun between Jupiter and Neptune have long represented one of
the enduring mysteries of astrophysics.
Because of their dual nature, they take their name from the creature in Greek mythology whose head and
torso are human and legs are those of a horse.
"Just like the mythical creatures, the centaur objects seem to have a double life," said James Bauer of
NASA's Jet Propulsion Laboratory in Pasadena, Calif. and the lead author of the paper.
Click on image to enlarge
This artist's concept shows a centaur creature together with asteroids on the left and comets at right.
Image Credit: NASA/JPL-Caltech
"Our data point to a cometary origin for most of the objects, suggesting they are coming from deeper
out in the solar system."
"Cometary origin" means an object likely is made from the same material as a comet, may have been an
active comet in the past, and may be active again in the future.
The findings come from the largest infrared survey to date of centaurs and their more distant cousins,
called scattered disk objects.
NEOWISE, the asteroid-hunting portion of the WISE mission, gathered infrared images of 52 centaurs and
scattered disk objects. Fifteen of the 52 are new discoveries.
NEOWISE can tell whether a centaur has a matte and dark surface or a shiny one that reflects more light.
The puzzle pieces fell into place when astronomers combined the albedo information with what was already
known about the colors of the objects.
Visible-light observations have shown centaurs generally to be either blue-gray or reddish in hue.
A blue-gray object could be an asteroid or comet.
Centaurs and scattered disk objects orbit in an unstable belt. Ultimately, gravity from the giant
planets will fling them either closer to the sun or farther away from their current locations.
"Comets have a dark, soot-like coating on their icy surfaces, making them darker than most asteroids,"
said the study's co-author, Tommy Grav of the Planetary Science Institute in Tucson, Ariz.
"Comet surfaces tend to be more like charcoal, while asteroids are usually shinier like the moon."
The results indicate that roughly two-thirds of the centaur population are comets, which come from the
frigid outer reaches of our solar system. It is not clear whether the rest are asteroids.
The centaur bodies have not lost their mystique entirely, but future research from NEOWISE may reveal
their secrets further.